Hereinafter, as an example of a conventional induction heating device, a description is provided of an induction heating cooker in which a heating coil generates a high-frequency magnetic field and eddy current produced by electromagnetic induction heats a load, such as a pan, with reference to FIG. 6.
FIG. 6 is a diagram showing the circuit structure of a conventional induction heating cooker disclosed in Patent Document 1. Power source 51 is a 200V commercial power supply, i.e. a low-frequency AC power supply, connected to the input end of rectifier circuit 52, i.e. a bridge diode. Between the output ends of rectifier circuit 52, first smoothing capacitor 53 is connected. Between the output ends of rectifier circuit 52, a serially connected body of choke coil 54 and second switching element 57 is also connected. Heating coil 59 is faced to aluminum pan 61, i.e. an object to be heated.
As shown in FIG. 6, the part surrounded by the dotted line is inverter 50. The terminal on the low potential side of second smoothing capacitor 62 is connected to the negative terminal of rectifier circuit 52. The terminal on the high potential side of second smoothing capacitor 62 is connected to the terminal on the high potential side (collector) of first switching element (insulated gate bipolar transistor, hereinafter referred to as an IGBT) 55. The terminal on the low potential side of first switching element (IGBT) 55 is connected to the junction point between choke coil 54 and the terminal on the high potential side (collector) of second switching element (IGBT) 57. A serially connected body of heating coil 59 and resonant capacitor 60 is connected in parallel with second switching element 57.
First diode 56 (a first reverse conducting element) is connected in anti-parallel with first switching element 55. Second diode 58 (a second reverse conducting element) is connected in anti-parallel with second switching element 57.
Snubber capacitor 64 is connected in parallel with second switching element 57. A serially connected body of correction resonant capacitor 65 and relay 66 is connected in parallel with resonant capacitor 60. Fed into control circuit 63 are a detection signal from current transformer 67 for detecting the input current from power supply 51 and a detection signal from current transformer 68 for detecting the current through heating coil 59. Control circuit 63 also supplies signals to the gates of first switching element 55 and second switching element 57 and to the drive coil (not shown) of relay 66.
A description is provided of the operation of the conventional induction heating cooker structured as above. Power supply 51 is full-wave rectified by rectifier circuit 52 and the rectified power is supplied to first smoothing capacitor 53 connected between the output ends of rectifier circuit 52. First smoothing capacitor 53 works as a supply source for supplying high-frequency current to inverter 50.
FIGS. 7A and 7B are diagrams showing the waveforms in the respective parts of the circuit of the conventional induction heating device. FIG. 7A shows the waveforms at a high output of 2 kW. Waveform A1 shows a waveform of current Ic1 flowing through first switching element 55 and first diode 56. Waveform B1 shows a waveform of current Ic2 flowing through second switching element 57 and second diode 58. Waveform C1 shows voltage Vce2 generated between the collector and the emitter of second switching element 57. Waveform D1 shows drive voltage Vg1 applied to the gate of first switching element 55. Waveform E1 shows drive voltage Vg2 applied to the gate of second switching element 57. Waveform F1 shows current IL flowing through heating coil 59.
As shown in FIG. 7A, at an output of 2 kW, control circuit 63 outputs an ON signal having a drive period of T2 (approximately 24 μs) to the gate of second switching element 57 from time t0 to time t1, as shown by waveform E1. During this drive period T2, resonance occurs in a closed circuit formed of second switching element 57, second diode 58, heating coil 59, and resonant capacitor 60. The number of turns (40T) of heating coil 59 and the capacitance (0.04 μF) of resonant capacitor 60 are set so that the resonance cycle when pan 61 is made of aluminum is approximately ⅔ time of drive period T2 (approximately 16 μs). When the resonance frequency is set as f, the resonance cycle is 1/f, which is shown in FIG. 7A. Choke coil 54 stores the electrostatic energy of smoothing capacitor 53, as magnetic energy, in drive period T2 of second switching element 57.
Next, at time t1, i.e. the timing between the second peak of the resonance current through second switching element 57 and the time when the resonance current is set at zero next, at which the collector current is flowing in the forward direction of second switching element 57, the driving of second switching element 57 is stopped.
This operation turns off second switching element 57, thus rising the electric potential of the terminal of choke coil 54 connected to the collector of second switching element 57. When this electric potential exceeds the electric potential of second smoothing capacitor 62, second smoothing capacitor 62 is charged through first diode 56, and the magnetic energy stored in choke coil 54 is released. The voltage of second smoothing capacitor 62 is increased to 500V so as to be higher than the peak value (283V) of DC output voltage Vdc of rectifier 52. The boosting level depends on the conduction period of second switching element 57. The longer conduction period tends to generate a higher voltage in second smoothing capacitor 62.
In this manner, when resonance occurs in a closed circuit formed of second smoothing capacitor 62, first switching element 55 or first diode 56, heating coil 59, and resonant capacitor 60, the voltage level of second smoothing capacitor 62 working as a DC power supply is increased. This operation changes the cusp value (peak value) of the resonance current flowing through first switching element 55 shown by waveform A1 in FIG. 7A and the resonance route so that the cusp value of the resonance current flowing through second switching element 57 in which successive resonance is to occur is not zero or a small value as shown in waveform B1. Thus, high-output induction heating is performed on an aluminum pan, and the output can continuously be changed and controlled.
Then, as shown by waveform D1 and waveform E1 of FIG. 7A, control circuit 63 outputs a drive signal to the gate of first switching element 55, at time t2 after a pause provided after time t1 to prevent simultaneous conduction of first switching element 55 and second switching element 57. As a result, as shown in waveform A1, resonance current flows through a closed circuit formed of heating coil 59, resonant capacitor 60, first switching element 55 or first diode 56, and second smoothing capacitor 62, in a different route. Drive period T1 of this drive signal is set to a period substantially equal to T2. Thus, similar to conduction of second switching element 58, a resonance current having a cycle approximately ⅔ of drive period T2 flows.
Therefore, current IL as shown by waveform F1 of FIG. 7A flows through heating coil 59. The drive cycle of the first and second switching elements (the sum of T1, T2, and the pause) is approximately three times of the cycle of the resonance current. When the drive frequency of the first and second switching elements is approximately 20 kHz, the frequency of the resonance current flowing through heating coil 56 is approximately 60 kHz.
FIG. 7B shows the waveforms at a low output of 450 W. Although the details are omitted, the drive cycle is set shorter than the drive cycle at an output of 2 kW.
At activation, control circuit 63 turns off relay 66, and alternately drives first switching element 55 and second switching element 57 at a fixed frequency (approximately 21 kHz). At this time, the switching elements are driven in a mode in which the drive period of first switching element 55 is shorter than the resonance cycle of the resonance current. In other words, the drive-time ratio is minimized to provide the minimum output setting, and then gradually increased. During this time, control circuit 63 detects the material of load pan 61 based on the detection output from current transformer 67 and the detection output from current transformer 68.
When control circuit 63 determines that the material of load pan 61 is iron-based, the control circuit stops heating, turns on relay 66, and restarts heating at a low output. At this time, control circuit 63 drives first switching element 55 and second switching element 57 at a fixed frequency (approximately 21 kHz) at the minimum drive-time ratio again. The output is at the minimum at the beginning and is gradually increased to a predetermined value.
On the other hand, when control circuit 63 detects that the material of load pan 61 is not iron-based and a predetermined drive-time ratio is reached, the mode is changed so that the cycle of the resonance current is shorter than the drive period of first switching element 57, as shown in FIG. 7B. At this time, the drive period is set to provide a low output.
As described above, when a load having a high conductivity and a low magnetic permeability, such as aluminum and copper, is heated by a magnetic field generated by heating coil 59, the resonance current through first switching element 55 and second switching element 57 caused by heating coil 59 and resonant capacitor 60 has a cycle (2T1/3) shorter than the drive period (T1) of each switching element. As a result, current at a frequency of three times of the drive frequency of first switching element 55 and second switching element 57 can be supplied to heating coil 59 for heating. Further, choke coil 54, i.e. a booster, and second smoothing capacitor 62, i.e. a smoothing part, are provided to increase and smooth the voltage of smoothing capacitor 62, i.e. a high-frequency power supply, and increase the amplitude of resonance current in each drive period. Thus, even when, after the start of driving, the first cycle of the resonance current is completed, the second cycle is reached and thereafter, the resonance current having a sufficiently large amplitude can be continued.
In the conventional induction heating cooker structured as above, load detection for determining whether the load is made of a material having a high conductivity and a low magnetic permeability, such as aluminum, or an iron-based material is accurately made at a low output. Thus, turning on/off the relay can switch the resonant capacitor and thus allows induction heating in which large heating output can efficiently be obtained according to the material of the load.
Further, Patent Document 2 discloses a method in which switching between a full-bridge circuit system and a half-bridge circuit system according to whether the load is a magnetic pan or a non-magnetic pan eliminates the need of a switching relay for both of the magnetic pan and the non-magnetic pan.
However, in the conventional structure of changing the capacitance of the resonant capacitor according to the material of the load as shown in Patent Document 1, a complicated structure, including a relay having a high withstand voltage for switching the resonant capacitor, is necessary for heating both of a load material having a high conductivity and a low magnetic permeability, such as aluminum, and a load of an iron-based material. Further, unless the capacitance of the resonant capacitor is set appropriately for heating aluminum or the like and switched, the following problem arises. Particularly in heating an iron-based load having a low conductivity, small capacitance of the resonant capacitor increases the drive frequency of the switching elements and the voltage to be applied to the switching elements. This phenomenon increases the loss of the switching elements and makes it difficult to provide sufficient output.
In the conventional structure shown in Patent Document 2, when the device is set appropriately for heating a material having a low conductivity, such as an iron-based material, and attempts to obtain high output in heating a material having a high conductivity and a low magnetic permeability, such as aluminum, the small equivalent resistance of the resonance circuit including the load is assumed to considerably increase the rated current of the inverter. Inversely, when the resonance circuit is set appropriately for heating a material having a high conductivity and a low magnetic permeability, such as aluminum, the maximum output power (hereinafter referred to as maximum heating output) of the resonance circuit is decreased, and the targeted heating output cannot be obtained with a material having a low conductivity. Thus, it is difficult to heat materials at a practical level, ranging from a material having a high conductivity and a low magnetic permeability, such as aluminum and copper, to a material having a low conductivity, such as a magnetic material.    [Patent Document 1] Japanese Patent No. 3460997    [Patent Document 2] Japanese Patent No. 2816621